The present invention relates, generally to a manner by which to facilitate selection of handoff of communications with a mobile station by a serving communication station to a target communication station, such as the serving and target base transceiver stations of a cellular communication system. More particularly, the present invention relates to apparatus, and an associated method, by which adaptively to select the threshold at which the handoff of communication is requested. Because of the adaptive selection of the threshold at which the request is initiated, an appropriate threshold is selectable irrespective of the communication characteristics of the cells or other geographical areas defined by the serving and target communication stations.
A communication system is formed, at a minimum, of a sending station and a receiving station interconnected by way of a communication channel. The communication channel provides a communication path by which information originated at the sending station is communicated to the receiving station. Communication systems have been developed, and implemented, to communicate a wide variety of types of information between a large variety of types of sending and receiving stations.
Advancements in communication technologies have permitted new types of communication systems to be developed and implemented. A radio communication system is exemplary of a type of communication system. In a radio communication system, the sending station, a radio transmitter, transmits information upon a radio communication channel to a receiving station formed of a radio receiver. The radio communication channel is defined upon a radio link formed between the radio transmitter and receiver. The conventional need for a wireline connection to interconnect the sending and receiving stations is obviated due to the formation of the radio communication channel upon a radio link. A radio communication system permits, as a result, increased communication mobility. Sending and receiving stations of a radio communication system are operable to send and to receive information, respectively, at positions in which communications by a conventional wireline system would be impractical or impermissible.
A cellular communication system is a type of radio communication system which provides multiple numbers of users concurrently to communicate therethrough. Generally, such a communication system is referred to as a cellular communication system as the geographical area over which the cellular communication system is installed is divided into regions, referred to as cells. The cells are each defined by a fixed-site transceiver, referred to as a base transceiver station. The base transceiver stations are positioned at locations such that the cells defined thereby encompass the geographical area in which the cellular communication system is to be operable.
Radio communications are effectuable with a mobile station positioned at any location within the area encompassed by the cellular communication system. Generally, a mobile station communicates with the base transceiver station which defines the cell in which the mobile station is positioned. If the mobile station moves to be repositioned at another cell, a procedure, referred to as a handoff, is effectuated. Handoffs of communications are sometimes also effectuated due to channel traffic conditions or channel communication conditions irrespective of the repositioning of the mobile station.
In a handoff procedure, communications of the mobile station, initially with a first base transceiver station, referred to as the serving station, are transferred to another base transceiver station, referred to as a target station. Once the transfer is completed, subsequent communications of the mobile station are carried out with the target base transceiver station. Successive handoffs, from one base transceiver station to another, can occur as the mobile station travels through successive cells.
Handoff of communications from a serving base station to a target base station permits communications by a mobile station to continue even though the mobile station travels out of communication range with the base station which initially forms the serving base station. When in an active communication session, uninterrupted communications are generally possible with a mobile station as a handoff is effectuated from a serving base station to a target base station. And, the handoff occurs both without the active intervention of a user of the mobile station as well as, generally, also without the knowledge of the user that a handoff has occurred.
In different cellular communication systems, different manners are utilized by which to select when to effectuate handoff of communications with a mobile station from a serving base station to a target base station. In some systems, determination of when to initiate handoff of communications is performed entirely at the network infrastructure portion of the communication system. In other systems, at least some of the information utilized in the determination of when to initiate handoff of communication is determined at, or otherwise provided by, the mobile station. In some cellular communication systems, a procedure referred to as MAHO (mobile-assisted handoff) is utilized in the determinations of when to initiate handoff of communications.
In an exemplary MAHO procedure, measurement is made at selected intervals, by a mobile station of signal indicia of signals transmitted by the serving base station and one, or more, potential target base stations. The signal indicia is formed, for instance of signal strength measurements of the signals. Responsive to such measurements a report is made by the mobile station to the network infrastructure to request, when appropriate, a handoff of communications from the serving base station to a selected target base station. Thereafter, handoff of communications is selectably effectuated.
Communication characteristics might differ, however, in differing ones of the cells. In some cells, for instance, relatively low levels of fading are exhibited. In other cells, conversely, relatively high levels of fading might be exhibited.
Conventionally, uniform thresholds are utilized in the handoff procedures irrespective of the communication characteristics of the cells defined by serving and potential target base stations. However, due to the different communication characteristics of the different cells, the uniform thresholds might cause selection of initiation of a handoff of communications when a handoff is not necessary. And, also, selection might be made not to initiate handoff of communications when a handoff is necessary. If a manner could be provided by which adaptively to select the thresholds, responsive to communication characteristics, used in handoff procedures, improved communications in a cellular communication system would be possible.
It is in light of this background information related to handoff procedures in a radio communication system that the significant improvements of the present invention have evolved.
The present invention, accordingly, advantageously provides apparatus, and an associated method, by which to facilitate selection of handoff of communications with a mobile station by a serving communication station to a target communication station.
Through operation of an embodiment of the present invention, a manner is provided by which adaptively to select the threshold at which the handoff of communications is requested. An appropriate threshold is selectable irrespective of the communication characteristics of the geographical area in which the mobile station is positioned. Selection of the threshold to be used in the handoff procedures is responsive, in part, to the communication characteristics of the geographical area is defined by the serving and target communication stations. By adaptively selecting the threshold at which the request is initiated, an appropriate threshold is determined thereby to ensure that selection of the handoff of communications between serving and target communication stations is properly made.
If the communication characteristics exhibit minor amounts of fading, the threshold is set to be of appropriate levels. And, when the communication characteristics exhibit significant amounts of fading, the threshold is correspondingly set to be of other appropriate levels.
In one aspect of the present invention, a manner is provided by which to perform adaptive handoff of communications between a serving communication station and a target communication station. By performing the adaptive handoff of communications of an embodiment of the present invention, unnecessary handoffs of communications are reduced while also maintaining a possibility of an inadvertent communication interruption, such as a dropped call in a cellular communication system. Indications of the positioning of the mobile station and indications of values of selected RF (radio frequency) signal data are utilized to adjust dynamically a handoff threshold. Values of indications of the location at which the mobile station is positioned are requested only when an initial indication that a handoff of communications should be requested occurs. Thereby, continuous tracking of the position of the mobile station along with the corresponding privacy concern, is obviated.
Problems associated with conventional use of a fixed, nonadjustable handoff threshold are avoided. Conventionally, if the threshold is selected to be of too small value, unnecessary handoffs result. Unnecessary handoffs increase the amounts of signaling generated during operation of the communication system. And, if the value of the fixed threshold is selected to be too large, a necessary handoff might not be performed, resulting in an inadvertent communication termination. By dynamically altering the value of the handoff threshold responsive to, variously, the position at which the mobile station is located and RF signal statistics, the number of unnecessary handoffs which are performed is reduced while maintaining a low probability of an inadvertent communication interruption, all in contrast to conventional handoff procedures.
In another aspect of the present invention, a hysteresis threshold formed of a set of values is dynamically selected. The values are selected according to a mapping of the position at which the mobile station is located and according to mean and standard deviations of measured RF propagation data.
In one implementation, the handoff threshold is adaptively selected for communications in a cellular communication system installed throughout a geographical area. Signal strength measurements are made by a mobile station of signals, such as pilot signals, generated by a serving base station and one or more potential base stations. When differences between the values of the signals generated by the serving and target base stations exceed initial threshold values, a preliminary indication is made that a handoff of communications should be effectuated. Once such an initial determination is made, a request is then made to determine mobile station indicia, of both position and velocity, of the mobile station. If the mobile station is not moving, is or moving at a velocity beneath a selected level, a handoff request is generate responsive to the determinations made with respect to the initial threshold values. If, however, the mobile station is traveling at velocity greater than the selected value, analysis is further made of RF propagation data associated with the environment about the mobile station. Responsive to such additional information the hysteresis threshold values are altered. A determination is then made as to whether a handoff of communications should be performed by comparing values of the differences, earlier determined, between the values of signals generated by the serving and target base stations and the newly-decided threshold values. If a handoff of communications is needed, as determined by the subsequent comparison, a request for handoff of communications is requested.
In these and another aspects, therefore, apparatus, and an associated method, is provided for a radio communication system in which a mobile station is operable to communicate with a serving fixed-site communication station and to communicate with a target fixed-site communication station subsequent to effectuation of a communication handoff thereto. Selection of when to initiate the effectuation of the communication handoff is facilitated. A movement determiner is coupled to receive indications of differences between signal indicia associated with signals generated by the serving fixed-site communication station and signal indicia associated with signals associated with signals generated by the target fixed-site communication station. The movement determiner is selectably operable at least when the indications of the differences are beyond a first threshold set to determine movement indicia associated with the mobile station. A second threshold set calculator is coupled to receive indications of the movement indicia determined by the movement determiner. The threshold set calculator calculates second threshold set values when the movement indicia indicates selected movement of the mobile station. A second threshold set comparator is coupled to receive indications of the second threshold set values and indications of the differences between the signal indicia associated with the signals generated by the serving and the target fixed-site transceivers. The second threshold set comparator compares differences between the signal indicia and the second threshold set values. A handoff request selector is coupled to the second threshold set comparator. The handoff request selector selectably requests handoff of communications to the target fixed site communication station responsive to comparisons made by the second threshold set comparator.
The present invention will be better understood when read in light of the accompanying drawings which are described in the detailed description hereinbelow and in light of the claims appended hereto.